In the conventional processes for producing polyurethanes in the form of coatings using blocked isocyanate compounds, high-temperature baking is required. Therefore, in coating thin steel sheets, there arise such problems as deformation of the steel sheets themselves, melting of solders and excessive energy consumption.
For solution of these problems, there have been proposed such deblocking temperature lowering catalysts for blocked isocyanate compounds as tetrabutyl-1,3-diacetoxydistannoxane, tetrabutyl-1,3-diphenoxydistannoxane, tetrabutyl-1,3-dibenzyloxydistannoxa ne, dibutyltin oxide, dibutyltin dilaurate and tetrabutyltin as disclosed in, for example, Japanese Patent Publication No. 18877/1969 and Japanese Patent Applications laid open (Kokai) under Nos. 42528/1977, 138434/1978 and 138435/1978. However, the effects of these compounds are still unsatisfactory. More efficient catalysts are demanded.